This invention generally relates to eyeglass frames and is specifically concerned with an eyeglass frame having an improved shock absorbing T-wire for mounting a safety lens.
Safety eyeglass frames for mounting prescription lenses are known in the prior art. Like regular eyeglass frames, such safety frames generally comprise a pair of lens-retaining rims connected by a bridge which rests on the upper portion of the wearer""s nose when the glasses are worn. Temples hingedly connected to the sides of the rims secure the lenses in front of the eyes of the wearer.
Safety eyeglasses differ from ordinary eyeglasses in that the lenses must effectively resist being dislodged from the rim in the event that a shock or impact force is applied onto the front surface of the eyeglass lenses. Consequently, prescription eyeglass rims formed from wire having a T-shaped cross-section were. adopted in the prior for use in safety glasses. Such T-wire rims include a rim portion having a centrally disposed, inner rib that protrudes upwardly from the inner rim surface at a right angle. The inner rib extends into a groove centrally disposed around a generally straight outer wall of the lens. The interfitting rib and groove arrangement creates a stronger connection between the lens and the surrounding rims, making it less likely that the lens will dislodge upon the application of an impact or shock force.
While such prior art T-wire rims have generally worked well for their intended purpose, the applicant has noted that the conventional T-wire design is much more prone to failure when applied to thinner lenses. Recent advances in polycarbonate chemistry have led to transparent plastic materials which advantageously combine the properties of higher shock resistance, and a higher index of refraction. Such new plastics will soon allow the thickness of safety prescription lenses to be reduced to only 2 millimeters. While the use of such thinner lenses advantageously reduces the weight of the eyeglasses and enhances their overall appearance by giving them the trim, lightweight look of ordinary prescription glasses, it also results in a weakened connection between the T-wire and the outer edges of the lenses, and promotes the occurrence of cracking in the lens in the vicinity of the centrally-disposed groove. Surprisingly, the applicant has observed that such dislodgment and cracking are not caused solely by the reduced thickness of the lens in the vicinity of the lens-rim joint, but instead from the generation of hoop stresses between the outer surface of the lens and inner surface of the T-wire in response to a sharp impact force on the outer surface of the lens. The applicant has determined that such hoop stresses are caused by the generally convex profile of the front surface of lens, which is present even when it is non-prescription, in combination with the increased flexibility of the lens due to its smaller thickness. These two geometries increase the tendency of the outer walls of the lens to momentarily flex outwardly in a radial direction in response to shock in what may be termed an xe2x80x9coil canxe2x80x9d effect. This momentary radial expansion in turn causes the T-wire to tighten so that the rim portions on either side of the rib of the T-wire momentarily apply powerful wedging forces to the groove that circumscribes the outer wall of the eyeglass lens. The resulting wedging forces between the T-wire and the lens groove may generate cracks at the end of the groove which may cause fracture and dislodgment of the lens in an accident situation. Even if the lens is not dislodged, the resulting fractures may necessitate replacement of the lens. The applicant has further observed that the increased tendency of such thinner lens to bend in response to shock or impact force can also result in slippage between the relatively short rib of the rims and the groove in the lens. Such slippage is another factor that can result in dislodgment of the lens from the rim.
Clearly, what is needed is an improved eyeglass frame having a T-wire rim which is capable of securely gripping the outer walls of an eyeglass lens during an accidental impact to the lenses without slippage, yet does not generate the wedging forces in the vicinity of the grooves circumscribing the lens that promote cracking and fracture. Ideally, the T-wire of the eyeglass frames should be compatible for use with lenses having a conventionally shaped straight outer walls circumscribed by a groove of conventional shape to obviate the need for new grinding or shaping techniques for safety lenses. Finally, it would be desirable if the T-wire of the frame were easily and inexpensively manufactured with existing machinery.
Generally speaking, the invention is an eyeglass frame for mounting a lens that overcomes the aforementioned shortcomings associated with the prior art. The eyeglass frame of the invention is particularly adapted for mounting a lens having a straight outer wall circumscribed by a groove, and comprises a T-wire including a centrally disposed rib portion receivable in the lens groove, and a pair of oppositely extending rims flexibly cantilevered from the rib portion. Each of the rims includes a retaining wall that engages opposite ends of the outer wall of the lens in line contact. Each of the retaining walls is shaped such that they flex toward the outer lens wall in response to momentary hoop stresses generated by shock to the lens in order to absorb the same.
In the preferred embodiment, the retaining walls of each of the oppositely extending rims are tapered at an angle between the rib portion and their outer edges. The taper angle of each of the retaining walls of the rims is preferably between about 10xc2x0 and 20xc2x0 relative to a line orthogonal to a central axis of the rib portion. When a momentary hoop stress is created by shock directed toward the front of the lens, each of the flexibly cantilevered rims flexes toward the lens, thereby reducing the taper angle and increasing surface contact between the outer edges of the retaining walls of the rims and the outer wall of the lens. After the shock abates, the cantilevered rims flex back into line contact with the lens.
To further increase the resistance of the frame to lens slippage when the lens is exposed to shock or impact forces, the rib portion includes a rib which projects from the rims a distance greater than the rims project from the rib portion. The depth of the groove is greater than the distance than the rib projects from the rims so that the distal end of the ribs does not come into contact with the bottom of the groove even during the application of shock to the lens. Sidewalls of the ribs are tapered with respect to the lateral walls of the groove to further eliminate the possibility of engagement and stress between the rib and the walls of the groove.
Finally, the joint between the rib portion and the cantilevered rims is rounded to facilitate flexible movement between these components.